Pulse Width Modulation (PWM) models an analog input signal as a sequence of pulses having a fixed amplitude and period but having a variable pulse width. The time integral of the PWM signal closely approximates the analog input signal.
Many digital amplifiers and some analog amplifiers use PWM to convert incoming audio signals to a pulse train and then increase the amplitude of the pulse train in order to amplify the input signal. Analog PWM is performed by analog circuits that provide a continuously variable switching point. That is to say, the pulse widths of the pulses produced by an analog PWM system are continuously variable within the period of the switching signal. In digital PWM, however, the switching point varies in discrete steps defined by the sampling signal. Typically, the frequency of the sampling signal (the sampling frequency) is much greater than the frequency of the switching signal (the switching frequency). Because the pulse width may vary only in discrete steps, digital PWM signals suffer from quantization noise and may require noise shaping or dithering to reduce at least the apparent quantization noise. Multi-bit output noise shaping may be performed at the switching frequency. This, however, is a non-linear process because the input waveform changes after the modulator has determined the preferred output pulse width. To overcome the error associated with this process, a linearizer may be used, but the resulting complexity of the modulator greatly increases its cost.
Digital PWM also suffers from minimum and maximum pulse width limitations. In order to control an output stage accurately, the pulse width is desirably held to a minimum number of clock cycles. Otherwise, the output stage can not track the desired PWM output signal. These minimums ultimately limit the output power attainable with a given power supply voltage. Furthermore, pulses greater than the maximum pulse width may unduly burden the power supply or require excessive cooling of the output transistors.
There is therefore a need for a modulator for digital amplifiers that overcome the limitations associated with digital PWM-based amplifier systems.